Process for the production of exceptionally-clean steel



Nov. 29, 1966 T. E. PERRY ETAL 3,288,589

PROCESS FOR THE PRODUCTION OF EXCEPTIONALLY-CLEAN STEEL Filed June 18,1962 2 Sheets-Sheet 1 INVENTORS JOHN A. RINEBOLT THOMAS E. PERRYRODERICK J. PLACE BY SIDNEY W. POOLE 4 AT R N E Y Nov. 29, 1966 T. E.PERRY ETAL 3,288,589

PROCESS FOR THE PRODUCTION OF EXCEPTIONALLYCLEAN STEEL Filed June 18,1962 ZSheets-Sneeta I o2 -22 RI .0 LL! 8 O O Y z 9 *2 0%" "P i N 8mg "-3'l- O "N Lk -.u, Ian. 55 o|- -w a. T501 0 0 CPU 0 o s w .4 cm 3 -LLI N0- n: 8 Qca ||||||||||||3 oooo oooooooo'j -uomgvmommvm NNN- INVENTR$-ATTORRNEY United States Patent f 3,288,589 PRUCESS FOR THE PRODUCTION OFEXCEPTHONALLY-CLEAN STEEL Thomas E. Perry, North Canton, John A.Rinebolt, Canton, Roderick J. Place, Massillon, and Sidney W. Poole,Canton, Ohio, assignors to Republic Steel Corporation, Cleveiand, Ohio,a corporation of New Jersey Filed June 18, 1962, Ser. No. 203,091 21Claims. (Cl. 7512) This invention relates to a process for theproduction of exceptionally-clean steel. More specifically, it relatesto the production of exceptionally-clean steel by a first step ofpreparing an electrode-ingot of an open steel, such as an air-melt in anelectric arc furnace, and thereafter, vacuum melting saidelectrode-ingot in a conconsumable electrode vacuum furnace.

In various types of steel to be heat-treated to high hardness andstrength levels, such as ball-bearing steel, missile steel, etc.,cleanliness of the steel, that is freedom from inclusions, is essential.Inclusions cause Weak spots, e.g., internal notches, in the steel. Forexample, when the steel is used in ball-bearings, cleanliness of thesteel is not most critical. Inclusions cannot be tolerated in such steelsince considerable pressures are exerted on very small areas on whichthe ball-bearings must rest or on which they must support considerableWeight.

When inclusions are present in the steel, the resistance to propagationof cracks or notches is lowered. In steels used for missile production,weight is an important factor. Therefore, the ordinary practice of usingexcess steel as a safety factor to provide for contingencies where apiece of steel may be weakened by inclusions cannot be tolerated. Steelsof high strength-to-weight ratios are required. Consequently, it isessential that a steel of extreme cleanliness or freedom from inclusionbe used for this purpose to insure good notch properties in the steel.

In missile and jet engines steels it is also essential to have toughnessand ductility as well as high tensile strength. High tensile strengthcan be obtained by increasing the carbon content. However, thisgenerally means a sacrifice in the toughness and ductility. Therefore,it is necessary to have good notch properties by avoiding inclusions inthe steel, and to have optimum tensile strength without increasing thecarbon content. These properties are effected, together with goodtoughness and ductility, by the process of this invention for theproduction of a very clean steel.

In previous attempts to produce exceptionally clean steel or steelsubstantially free from inclusions, any attempts to effect carbondeoxidation of the steel while in contact with refractories have beenunsuccessful. Refractories interfere with the production of clean steelfor a number of reasons: (1) spalling of the refractories results inpieces or particles of the refractory material getting into the melt;(2) some carbon acts to reduce the refractories as well as to deoxide inthe iron thus making the carbon control difficult; and (3) as the meltis reduced in oxygen content beyond a general level of about 15 p.p.m.,oxygen is fed back into the melt from the oxides in the refractories. Inthe presence of refractories, therefore, the deoxidation cannot beeffected below 5-10 parts of oxygen per million. However by the practiceof this invention, it is found possible to deoxidize to less than 5parts of oxygen per million parts of steel.

In accordance with the present invention it has now been found thatsurprisingly good results are obtained in the production ofexceptionally clean steel by the process wherein the electrode forutilimate consumption in a consumable electrode vacuum process isprepared by a first step of melting which results in an open steel,i.e., a

3,2385%? Patented Nov. 29, 1966 non-killed steel, for example byair-melting in an electric arc furnace a charge of scrap iron and anyalloying elements as well as any conditioning materials to be used suchas slag-forming materials. The steel at tap contains absolute minimumamounts of the strong metallic deoxidizers, such as Al, Ti, etc., and asa result is relatively high in total gas content. The resultant ingot isformed into the desired size and shape either by using an ingot rnold ofthis desired size and shape, or by having the ingot shaped by forging,rolling or other means into the desired electrode form.

The carbon content of the charge is adjusted by using scrap steel orother iron charge of suflicient carbon content or by the addition ofcoke or other carbonaceous materail to gives an ingot of sufficientcarbon content as to supply carbon for the carbon deoxidation reaction1n the subsequent consumable electrode vacuum melting step.

If a reduced pressure of microns or less of mercury, advatange-ously 10microns or less, preferably 3 microns or less, can be maintained in theinital consumable electrode vacuum melting, then the carbon deoxidationreaction is satisfactorily effected to produce satisfactoryexceptionally-clean steel. Where very efficient exhaust pumps areavailable to maintain such reduced pressure on the initial vacuummelting, either by use of a single super-efficient pump or by employinga number of pumps to maintain this condition, then a second vacuummelting step is not necessary. In cases where the exhaust pumps are notsufiiciently efiicient, or the amount of oxygen to be removed is of suchgreat amount that it is difiicult, because of the large amounts of gasbeing given off, to maintain the reduced pressure below the statedamount, then a second vacuum meltmg step maintained below this limitwill produce the desired result. In such case, the first vacuum meltingstep can be effected at a considerably higher pressure, that is whatevercondition is effective in removing excessive gas so as to permitsubsequent melting conditions as described above. It is also possible toslow down the rate of gas formation and thereby facilitate maintainingthe desired low pressures by slowing down the melting rate, e.g., bylowering the power input and thus reducing the burn-off rate.

In cases where the amount of gas eventually being given off by thecarbon deoxidation reaction is the cause for difiiculty in maintaining areduced pressure of 100 microns or less, it is sometimes possible toreduce the amount of ultimate gas formation by an intermediate partialdegassification in the ladle, or by vacuum lift degassing, or bybubbling argon through the molten metal while it is still in the furnaceor ladle. Stream and ladle degassifiers suitable for intermediatedegassification are illustrated on pages 3689 of Republic Alloy Steels(copyrighted 1961). Suitable vacuum lift degassers are illustrated inUS. Patent 3,033,550. Such intermediate treatment can often avoid thenecessity for repeating the vacuum melting step. Also as previouslyindicated the rate of gas formation can be controlled by adjusting thepower input or burn-off rate.

By the practice of this invention, it is possible to effect finaldeoxidation without the addition of strong metallic deoxidizers, such asaluminum or significant amounts of silicon or manganese. By using theconditions of this process, it is possible to effect the finaldeoxidation solely by the use of carbon. Because of the improvedcleanliness of the resultant steel and the lower dissolved gas level,the product has improved transverse ductility, better fatigueresistance, and better toughness. Moreover, this process can be used formost types of alloy steel by the addition of the desired alloyingmetals, and can also t be used for the production of carbon steel togive a cleaner steel with less non-'metallics and less gases.

In the vacuum melting step, the pressure is advantageously maintainedbelow microns of mercury, preferably below 3 microns. In the event thepressure rises substantially above 100 microns for any substantialperiod, it becomes necessary to perform a second vacuum melting duringwhich the pressure is maintained below the stated 100 microns.

In the air-melting or first melting step of the process of thisinvention various methods of melting which will give an open ornon-killed steel can be used. However, an electric arc furnace ispreferred because of available large capacities, and also because of thecapability of maintaining a deoxidizing type of slag and and thus aflexibility of composition. Also it gives better control'of sulfur inthe melt as compared to certain other types of furnaces.

The air-melting step, such as performed in electric arc, open-hearth,pneumatic, e.g., Linz-Donawitz process, induction melting, etc., isconducted in accordance with normal practice for the operation of suchfurnaces or melting operations except that when the melt is tapped, themelt is essentially free of strong metallic deoxidizers.

Carbon deoxidation takes place during the vacuum arc remelting as themetal is transferred across the arc and as it is maintained in themolten state.

It has been found that the process of this invention permits the use ofcarbon alone for the final deoxidation in the vacuum arc consumableelectrode furnace and that the amount of carbon required for suchpurpose can be calculated and easily controlled so as to give no morethan the desired carbon aim in the resultant products.

In the first melting step of this process, a standard electric arcfurnace such as presently in common commercial use can be used, such asdescribed on pages 482-484 of the United States Steel Corporationpublication, The Making, Shaping and Treating of Steel (copyrighted1951). To such a furnace, there is charged suitable scrap together withsufficient carbon to insure that the melt contains a carbon contentabove the required specification level. Alloying elements such as nickeland molybdenum, where desired in the ultimate product, can be charged atthis time as these elements are not oxidized in an iron base melt underthe oxidizing conditions imposed during meltmg.

After the charge is melted, oxygen is introduced into the molten metal.This can be done either by blowing the melt with a highly concentrateddry oxygen gas using a suitable lance for introducing oxygen at thesurface of the molten metal, or by adding iron oxide. The oxygenaddition provides temperature uniformity and the elimination of metallicdeoxidizers such as aluminum, silicon, etc.

During the finishing period of the first melt a carbide slag is used asthe finishing slag in order to refine the steel and to maintain aminimum oxygen level consistent with the carbon content and the bathtemperature.

The carbon content of the bath is adjusted to be desired carbon aim justprior to tapping the bath. The carbon can be added in various forms. Forexample, if chromium is required in the ultimate product, the carbon andchromium can be added in the form of pig iron either by itself or tosupplement any ferrochrome that is added.

The amount of carbon added is calculated roughly on the basis ofapproximately 100150% of the theoretical amount required to convert tocarbon monoxide the oxygen present in the iron, and to supply thedesired carbon aim for the ultimate product. The calculated amount ofcarbon to be added should allow also for any carbon already present inthe iron. Therefore,'the amount of carbon to be added, regardless of theform of addition, can be calculated roughly as the carbon aim plus 1l.5times the stoichiometric amount required to reduce the oxygen present tocarbon monoxide, minus the amount of carbon already present in the iron.

The molten metal is tapped and poured into suitable ingot molds. Sincethere are no strong metallic deoxidizers present and there is verylittle residual manganese, the steel is poured open and gas evolutionproceeds during solidification in the ingot mold.

The resultant ingot can be used as such as an electrode or forged orotherwise shaped into the desired electrode shape for use in theconsumable electrode vacuum furnace.

FIG. 1 of the accompanying drawings illustrates a schematic arrangementof a typical consumable electrode vacuum furnace that can be used in thepractice of this invention.

FIG. 2 shows the Charpy impact transition curves for a steel made by theprocess of this invention and for steels made by alternate methods.

In operating such a consumable-electrode vacuum furnace, the atmosphereis exhausted, preferably down to the pressure of 1-2 microns. As theelectrode is taken to a temperature at which melting starts, thepressure is desirably maintained at less than microns, preferably lessthan 10 microns, depending upon the capacity of the pumping equipment toremove the gases given off. If the capacity of the pumping equipment isnot sufficient to remove the gases as given off, particularly where theamount of oxygen in the steel is excessive or the melting rate is veryfast, the pressure may rise as high as 600 or 1,000 microns, or evenhigher. In such cases a remelting in the vacuum furnace is desirable toproduce exceptionally clean steel. As an alternative, the melting ratecan be reduced, so as to decrease the rate of gas evolution, by reducingthe power input.

The electrode is thus arc-melted into acopper crucible. If the productfrom this melt is to be remelted, this ingot is forged or otherwiseshaped into a new electrode which is subsequently remelted under areduced pressure of no more than 100 microns (0.1 mm.), advantageouslyno more than 10 microns (0.01 mm.) of mercury, and preferably 3 micronsor less.

In cases where the pressure during the first melting in the vacuumfurnace has exceded the desired amount, the resultant ingot may haveholes, particularly at the top of the ingot.

In the vacuum melting operation, the vacuum can be effected by amechanical pump in the early stages for preliminary removal of theatmosphere and then by one or more oil diffusion pumps to morecompletely exhaust gases and produce and maintain t he desired vacuum.

The type of equipment and method for effecting the vacuum arc meltingcan be of various types normally used for such purposes. Typical of atype of apparatus suitable for this purpose is that shown in PatentsNos. 2,727,936 issued December 20, 1955; 2,818,461 issued December 31,1957; and in application Serial No. 698,256, filed by Robert I. Garmy,on November 22, 1957, now Patent No. 2,973,452.

In the apparatus shown schematically in the accompanying drawing,consumable electrode 1 is held in position by supporting means (notshown) but positioned in a region above copper crucible 2. The coppercrucible is cooled by water flowing in water inlet 3 and out wateroutlet 3' and circuilating between the copper crucible and the outersupporting shell 4. This copper crucible acts as a receptacle for themelt 5. Power supply 6 feeds current through conductor 7 and throughpower tube 8 to electrode 1 and through conductor 9 to the coppercrucible. The arcing effect between the melt in the crucible and theconsumable electrode is shown by the jagged lines connecting theelectrode 1 and the melt 5. The position of the consumable electrode isadjusted upwardly gradually to control the arcing as the level of 5 israised by additional melt. Vacuum pump 10 creates and maintains a vacuumon the furnace and exhaust gases are forced out through outlet 11.

It has been possible by the process of this invention to 5 6 reduce theoxygen content to levels below 5 parts per TABLE million. The ingotobtained after the vacuum melting at 1355 than 10 microns, Pmferably1655 than 3 microns, Process Percent Percent Mn Percent Si Percent Al issound and dense, and is suitable for purposes which require an extremelyclean steel having high strength, duc- 0.19 0.28 0.21 0.05 tility, andnotch toughness. It has also been found, ap- 016 (124 0.15 Traceparently because of the violent reaction between the car- 0-13 0.16 Q01Trace bon and the oxides in the metal, and the accompanying sweepingeffect of the resultant carbon monoxide, that As shown b the curves ofFIG. 2 the Char 1m act i IS an effefztwe fi m1 ogmfrogen i may listrength for a typical steel made acording to ti ie p i'acmany pmsent mthe esults m lower i f tice of this invention is much improved overthose made levels than would be the case if only vacuum remeiting bystandard electric arc furnace as Well as by a corre wasemploifedsponding subsequent vacuum melt Where Mn and Si had Theparticular effectiveness of this invention in producsen added in thepreliminary electric arc melt eXcsptiOnilHY clean Steel is demonstratedby results The following examples illustrate various modificationsobtained when steels produced according to this invention for practicingh process f hi invention These examare tested according to the 1Kinclusion rating described pies ar i te d d erel as illu tration a d reot to be in, Tentative Recommended Practice for Determining interpretedas limiting the scope of the invention or the the Inclusion Contents ofSteel, designation B 45-60 T manner in which the invention can bepracticed. Unless hi h ear in paragraph 3, pages 105-118, of thespecifically indicated otherwise, parts and percentages are 1960Supplement for the American Society for Testing glvfin as Parts andPflcentags y Weight- Material Standards. As illustrated hereinafter inthe ex- Example I i the Values for the Chart or table of page 112 i Acharge of 385 lbs. of scrap steel low in phosphorus me above supplemntgefmrany not exceed a Value 0 and sulfur plus 11 lbs. 8 ozs. of nickeland 3 lbs. of ferro 1 for any type of inclusion and in most cases thereare molybdenum together with 2 lbs. of Garbo as can no lncmslons or theyhave a value no greater than bon is added to an electric arc furnace ofstandard comwhen steels are produced according to the practice ofmsrcial design This is covered with a layer f 20 this invention. oflime, and 8 lbs. of spar to serve as slag covering. This For example,the following table shows the 1K rating charge i melted by applying120-140 volts and 1500- f a hinh i ld th steel (A) d by normal l 1800amps. for 70 to 90 minutes. Then the molten metal tric arc furnace inwhich Mn, Si and A1 are added as is decafhuriled y l Wing with pureoxygen The reusual, and a similar steel (A,/V) made by an electricsultant slag is run off and replaced with a finishing slag ar-c furnacefirst melt Without any addition of Mn or Si cpinpnsmg 20 lbs of lime and8 Spar 4 silica sand plus 3 lbs. coke fines. At this time a sample or Aland a Second Single Vacuum melt of the resultant of the melt isremoved'for analysis to determine Whether ingot at a riiduced Pressureof liss than 10 mlcrons of elements need to be added to obtain theultimate desired mercury, in accordance with the practice of thisinvenspecification. In this case, the chemical analysis shows m 40 thefollowing percentages:

0 Mn P s Si Ni Cr M0 Cu 1st Prelim .16 .14 .008 .015 .03 2. 95 .05 .48.04 Spec. Aim .1810 .010 .010 2 80to 1.50 to .45 to .19 Nil Max. Max.Nil 3. 10 1.70 .55

INCLUSION RATING AND GRAIN SIZE Additions of 9 lbs Fe-Cr and .13 lb.Fe-Mo are made to give the desired aims. Then the melt is tapped into an[J-K inclusion rating] open ingot of such dimensions as to give an ingotof 7" RC Sq. x 24" long x 350 lbs. This ingot is forged to ProcessSulphlde A Alumna B Silicate G Globulal D a 5%" rd. x 45" longelectrode, the skin ground off, and

T H T H T H T H the ingot prepared for remelting as an electrode in aconsunTigble eleltctrtzde1 vacuim fur1nac.

e resu an e ec ro e is p ace in a vacuum consum- A 0 L5 0 able electrodefurnace of commercial design and the at- I 0 0 0 0 mosphere is exhaustedto a reduced pressure of 1-2 microns. Then the electrode is melted bythe application FIG. 2 shows a series of Charpy impact transition of24-26 volts and 3800-4400 amperes for 120 minutes curves for high yieldstrength steels made by the following while maintaining a reducedpressure of 2 microns of methods: mercury and collecting the melt in thecopper crucible of A-Electric arc furnace air melt-conventional practicethe furnace. The resultant ingot is dense and has the with Mn and Siadded as usual. properties described above for A'/ V.

A/V-Electric arc furnace air melt with Mn and Si E l H added-remelted inconsumable-electrode vacuum arc mm? 8 furnace (single vacuum melt at 10microns). A charge of 350 lbs. of scrap steel low in phosphorusA/VElectric arc furnace air melt without Mn, Si and sulfur plus 36 lbs.of nickel, 16 /2 lbs. of cobalt, 1 lb. or Al addition-rernelted inconsumable-electrode vacu- 8 ozs. of ferro molybdenum together with 3lbs. of Carbo um arc furnace (single vacuum melt at less than 10 mi- 90as carbon is added to an electric arc furnace of crons). (According tothis invention.) standard commercial design. This is covered with a Thechemical analyses for C, Mn, Si and A1 of the layer of 20 lbs. of lime,and 8 lbs. of spar to serve as above products, which are also thoseshown in FIG. 2, slag covering. This charge is melted by applying120-140 are given in the following table. The analyses (not volts and1500-1800 amps. for 70 to 90 minutes. Then shown) for P, S, Ni, Cr andMo are approximately the the molten meta-l is decarburized by blowingwith pure same for the three products: oxygen. The resultant slag is runoff and replaced with a finishing slag comprising 20 lbs. of lime, and 8lbs. of spar, 4 lbs. silica sand plus 3 lbs. coke fines. At this time asample of the melt is removed for analysis to deter- .mine whetherelements need to be added to obtain the ultimate desired specification.In this case, the chemical 5 analysis shows the following percentages:

8 Example VI The procedure of Example III is repeated with similarsatisfactory results using as the consumable electrode in the vacuumremelting an ingot of open steel produced by an air-melt in anopen-hearth furnace.

Mn P S Si Ni Cr Mo 00 Va 1st Pre1im 33 16 006 02 8. 75 04 27 3. 90 2ndPrelim 45 13 006 012 02 9. 05 04 32 4. 00 Specification 44 to 010 010 8.O0 to 25 to 30 to 3. 75 t0 08 to 46 Nil Max. Max. Nil 10. 00 35 35 4. 1012 1 After refining slag is put on.

Final additions to give the desired aims are 1 lb. 12 02$. 15 FeCr and12 ozs. Fe-Va. Then the melt is tapped into an open ingot of suchdimensions as to give an ingot of 7" RC Sq. X 24" long x 350 lbs. Thisingot is forged to a 5 /2" rd. x 45" long electrode, the skin groundofi, and the ingot prepared for melting in the consumable-electrodevacuum furnace. The electrode is melted under conditions similar tothose of Example I with similar results.

Example III A charge of 144,000 pounds of scrap steel low in phosphorusand sulfur plus 15,000 lbs. of nickel and 6300 lbs. of cobalt plus 1200lbs. molybdenum oxide together with 2400 lbs. of carbon in the form ofground electrodes is added to an electric furnace of standard commercialdesign. This is covered with a layer of 3000 lbs. of lime and 1500 lbs.of spar which serves as a slag covering. This charge is melted byapplying approximately 360 to 380 volts and 10,000 amps which amounts to62,000 kw. hrs. for the whole heat. Then the molten metal isdecarburized by blowing with pure oxygen. The resulting slag is run offand replaced with a finishing slag comprising 3000 lbs. of lime, 1500lbs. of spar and 500 lbs. of coke fines and 100 lbs. of sand. At thistime a sample of the melt is removed for analysis to determine whetherelements need to be added to obtain the ultimate desired Example VII Theprocedure of Example III is repeated with similar satisfactory resultsusing as the consumable electrode in the vacuum remelting an ingot ofopen steel produced by a first melt in a basic oxygen (pneumatic)furnace wherein oxygen is blown downward vertically into the melt.

When the procedure of Example V is repeated with a vacuum applied in theinduction first melting, it is found that carbon deoxidation proceeds toan undesired extent in this first melting with the result that (-a) therefractory with which the melt is in contact is deoxidizedsimultaneously with the iron deoxidation, and (b) as the oxygen level inthe bath is being reduced, additional oxygen is being taken into thebath from the refractory. Consequently, it is desirable to perform thisinduction melting at approximately atmospheric pressures or at leastpressures not sufficiently reduced as to cause the disadvantages recitedabove.

While certain features of this invention have been described in detailwith respect to various embodiments thereof, it will, of course, 'beapparent that other modifications can be made within the spirit andscope of this invention and it is not intended to limit the invention tothe exact details shown except insofar as they are despecification. Inthis case, the chemical analysis shows the following percentages: fined1n the following claims.

0 Mn P I s Si Ni Cr M0 00 Va 1st Prelim .366 .15 .009 .008 02 7. 70 .16.29 3.14 o. 0 Specification .42 to 15 010 007 10 8. 75 to 25 to 30 to 3.to 08 to 44 Max. Max. Max. Max. 0. 25 35 .35 4. 00 12 Final additions togive the desired aims are 2475 lbs. of nickel, 1240 lbs. cobalt, 145lbs. ferro-molybdenum, 400 lbs. of ferrochromium, 175 lbs. of vanadiumplus 204 lbs. of carbon. Then the melt is tapped into open ingots ofsuch dimensions as to give ingots of 24" diameter and approximately 165"long x 20,000 lbs. These ingots are then used as the electrodes forconsumable vacuum melting using amperages between 5,000 and 30,000 amps.at 20-25 volts at a reduced pressure of less than 3 microns for asufiicient time in each case to complete the melt. Similar results areobtained as in Example I with respect to JK micro inclusion ratings. Theoxygen content of the product in each case is in the range of 1-4 partsper Example V The procedure of Example III is repeated with similarsatisfactory results using as the consumable electrode in the vacuumremelting an ingot of open steel produced by an air-melt in an inductionfurnace.

The invention claimed is:

1. A process for the preparation of an exceptionally clean steelcomprising the steps of:

(a) melting a charge of iron at atmospheric pressure in the absence ofan added metallic deoxidizer selected from the class consisting ofstrong deoxidizers of the class consisting of aluminum and titanium andsignificant amounts of weaker deoxidizers of the class consisting ofmanganese and silicon;

(b) adjusting the carbon content of the resultant melt while still inmolten form to an amount sufiicient to react With the oxygen content ofsaid melt and also to supply an additional amount of carbon required forthe ultimate carbon aim in the resultant steel,

(c) thereafter casting said melt into an ingot;

(d) melting said ingot as the consumable electrode in a vacuum,consumable-electrode electric arc furnace while the electrode region ofsaid furnace is maintained at a reduced pressure of no more than micronsof mercury.

2. A process of claim 1, in which said reduced pressure is no more than10 mircons of mercury.

3. A process of claim 1, in which the carbon adjustment comprises theaddition of carbon to said melt of an amount of carbon approximately1-1.5 times the stoichiometric amount required to react with the oxygencontent of said melt and also to supply any additional amount of carbonrequired for the ultimate carbon aim in the resultant steel.

4. The process of claim 3 in which said carbon is added as a high carbonferrochrome.

5. The process of claim 3 in which said carbon is added in the form ofpig iron.

6. A process of claim 3 in which said reduced pressure is no more thanmicrons of mercury.

7. A process of claim 1 in which the melt of said airmelting isdegasified under a reduced pressure of no more than 1,000 microns ofmercury, the resultant melt thereafter being cast into an ingot, and theresultant ingot remelted as a consumable electrode in a vacuumconsumable electrode furnace under a reduced pressure of no more than100 microns of mercury.

8. A process of claim 7 in which said reduced pressure in said vacuumconsumable electrode furnace is no more than 10 microns of mercury.

9. A process for the preparation of an exceptionally clean steelcomprising the steps of:

(a) air-melting a charge of scrap iron in an electric arc furnace in theabsence of an added metallic deoxidizer selected from the classconsisting of strong deoxidizers of the class consisting of aluminum andtitanium and significant amounts of weaker deoxidizers of the classconsisting of manganese and silicon;

(b) adjusting the carbon content of the resultant melt while still inmolten form to an amount suflicient to react with the oxygen content ofsaid melt and also to supply an additional amount of carbon required forthe ultimate carbon aim in the resultant steel;

(c) thereafter casting said melt into an ingot; and

(d) melting said ingot as the consumable electrode in a vacuum,consumable-electrode electric arc furnace While the electrode region ofsaid furnace is maintained at a reduced pressure of no more than 100microns of mercury.

10. A process of claim 9, in which the carbon adjust ment comprises theaddition of carbon to said melt of an amount of carbon approximately1-1.5 times the stoichiometric amount required to react with the oxygencontent of said melt and also to supply any additional amount of carbonrequired for the ultimate carbon aim in the resultant steel.

11. The process of claim 10, in which said carbon is added as a highcarbon ferrochrome.

12. A process of claim 10, in which said carbon is added in the form ofpig iron.

13. A process of claim 9, in which said reduced pressure is no more than10 microns of mercury.

14. A process of claim 9, in which said reduced pressure is no more than3 microns of mercury.

15. A process of claim 9, in which the initial melting of said scrapiron is completed under a layer of a carbide slag.

16. A process of claim 9, in which the initial melt of said scrap istreated by blowing with concentrated oxygen introduced under the surfaceof said melt.

17. A process of claim 9, in which the melting of said scrap iron iscompleted under a layer of a carbide slag, with the melted iron beingtreated with oxygen by blowing a jet of concentrated oxygen gas at thesurface of said melt.

18. A process of claim 9, in which the melt of said air-melting isdegasified under a reduced pressure of no more than about 1,000 micronsof mercury, the resultant melt thereafter being cast into an ingot, andthe resultant ingot remelted as a consumable electrode in a vacuumconsumable electrode furnace under a reduced pressure of no more than100 microns of mercury.

19. A process of claim 1, in which said melting at atmospheric pressureis effected in an induction furnace.

20. A process of claim 1, in which said melting at atmospheric pressureis effected in an open-hearth furnace.

21. A process of claim 1, in which said melting at atmospheric pressureis effected in a basic oxygen furnace.

References Cited by the Examiner UNITED STATES PATENTS 3/1965 Dagan 43OTHER REFERENCES DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, HYLAND BIZOT,

Examiners.

H. F. SATIO, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,288,589 November 29, 1966 Thomas E. Perry et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 1, line 24, strike out "not"; line 57, strike out in"; column 2,line 15, for "materail" read material column 4, line 41, for "exceded"read exceeded Signed and sealed this 12th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

1. A PROCESS FOR THE PREPARATION OF AN EXCEPTIONALLY CLEAN STEELCOMPRISING THE STEPS OF: (A) MELTING A CHARGE OF IRON AT ATMOSPHERICPRESSURE IN THE ABSENCE OF AN ADDED METALLIC DEOXIDIZER SELECTED FROMTHE CLASS CONSISTING OF STRONG DEOXIDIZERS OF THE CLASS CONSISTING OFALUMINUM AND TITANIUM AND SIGNIFICANT AMOUNTS OF WEAKER DEOXIDIZERS OFTHE CLASS CONSISTING OF MANGANESE AND SILICON; (B) ADJUSTING THE CARBONCONTENT TO THE RESULTANT MELT WHILE STILL IN MOLTEN FORM TO AN AMOUNTSUFFICIENT TO REACT WITH THE OXYGEN CONTENT OF SAID MELT AND ALSO TOSUPPLY AN ADDITIONAL AMOUNT OF CARBON REQUIRED FOR THE ULTIMATE CARBONAIM IN THE RESULTANT STEEL, (C) THEREAFTER CASTING SAID MELT INTO ANINGOT; (D) MELTING SAID INGOT AS THE CONSUMABLE ELECTRODE IN A VACUUM,CONSUMABLE-ELECTRODE ELECTRIC ARC FURNANCE WHILE THE ELECTRODE REGION OFSAID FURNANCE IS MAINTANED AT A REDICED MICRONS OF MERCURY.